Mechanical components, such as a gear or the like acting as a power transmission component in an automatic transmission, must have bending fatigue strength and toughness. Due to environmental issues, high dimensional accuracy has also been demanded in recent years in order to suppress noise during gear operation. In the past, case-hardened steels such as JIS SCr420 and SCM420 were often used as material for such gears. However, the growing trends toward less noise and more strength in automotive components have created demands, in terms of strength and dimensional accuracy, that cannot be fully met by conventional case-hardened steels.
Carburized gears undergo almost no mechanical processing and are used as is following carburizing treatment, or used after undergoing only low-temperature tempering following carburizing treatment. As a consequence, heat treatment distortion due to carburizing and hardening remains, which causes noise during operation.
In an attempt to meet such demands, a method disclosed in Japanese application publication H08-311607 subjects a steel, containing by mass 0.30% to 0.60% C, to a carburizing treatment to form a carburized layer. The steel is gradually cooled after the carburizing treatment, after which only the surface layer is subjected to induction hardening. In this method, because the core of the steel is not affected by induction hardening and is not subjected to quenching, the result is considerably less martensitic transformation and heat treatment distortion. However, the high C content leads to a problem of reduced toughness. Furthermore, the poor machinability of the material before carburizing is also a problem.